Apparatus and method for purifying exhaust gas of internal combustion engine

ABSTRACT

In an exhaust gas purifying apparatus provided with a lean NOx catalyst supporting a catalyst layer, which contains a NOx trapping component, on a honeycomb substrate formed so as not to cause an alkali attack, the invention prevents trapped NOx from being dissociated and exhausted during the time of a rich spike. A NOx reducing catalyst with the function of reducing NOx by a reductant in a rich or stoichiometric condition, e.g., a three-way catalyst, is disposed downstream of the lean NOx catalyst. In the case of increasing the amount of the NOx trapping component in the lean NOx catalyst to enhance a NOx trapping capability, even if a part of trapped NOx is dissociated during the time of the rich spike, the dissociated NOx can be reduced by the NOx reducing catalyst disposed on the downstream side.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and a method for purifyingexhaust gas exhausted from an internal combustion engine that isoperated in a lean atmosphere.

2. Description of the Related Art

A lean NOx catalyst is known as a catalyst for effectively purifyingnitrogen oxides (hereinafter referred to as “NOx”) contained in exhaustgas in which oxygen is also contained. One known example of the lean NOxcatalyst is a NOx trapping type catalyst that traps and reduces NOxcontained in exhaust gas with absorption, adsorption and otherreactions.

The lean NOx catalyst of the NOx trapping type contains, in a catalystlayer, a NOx trapping component and a NOx reducing component. Examplesgenerally used as the NOx trapping component are alkali metals, such aslithium (Li), sodium (Na), potassium (K), and cesium (Cs), which arehighly basic and belong to group IA of the Periodic Table. Also,examples generally used as the NOx reducing component are noble metals,such as platinum (Pt), rhodium (Rh), and palladium (Pd).

However, the lean NOx catalyst containing the alkali metal element has aproblem of causing an alkali attack phenomenon. The term “alkali attackphenomenon” means a phenomenon that the alkali metal in the catalystlayer drifts into a substrate. The lean NOx catalyst is usually of astructure in which the catalyst layer is supported on a honeycombsubstrate made of cordierite. The cordierite contains, as one component,silicon (Si) that tends to very easily bind with the alkali metal.Accordingly, when a heat load is applied to the lean NOx catalyst, thealkali metal drifts into the cordierite and the alkali attack phenomenonoccurs.

The occurrence of the alkali attack phenomenon reduces the amount of thealkali metal present in the catalyst layer and hence deteriorates theNOx trapping capability of the catalyst. Further, the strength of thehoneycomb substrate is reduced due to a resulting chemical change incomposition of the cordierite.

In order to suppress the alkali attack phenomenon, it has been proposedto form the honeycomb substrate by using a material that contains no Si(see, e.g., JP-A-10-165817 (abstract and claims)).

SUMMARY OF THE INVENTION

The alkali attack phenomenon can be suppressed by using the honeycombsubstrate that has a low affinity with the alkali metal. If the alkaliattack phenomenon does not occur, a reduction in the strength of thehoneycomb substrate resulting from the alkali attack phenomenon can beavoided. It is therefore possible to increase the amount of the alkalimetal supported on the lean NOx catalyst, and to enhance the NOxtrapping capability of the catalyst.

Meanwhile, the inventors have found that the following problem arises inthe case of increasing the amount of the alkali metal supported on thelean NOx catalyst and enhancing the NOx trapping capability of thecatalyst. More specifically, when the trapped NOx is reduced with theoperation atmosphere changed over to a stoichiometric or rich state,this process relatively easily causes a phenomenon that a part of thetrapped NOx is dissociated from the catalyst due to heat generated fromthe reduction reaction, etc. while it remains not reduced. Such a NOxdissociation phenomenon is apt to occur particularly in a state that alarge mount of NOx is trapped by the lean NOx catalyst, or in a statethat the temperature of exhaust gas is high and the NOx trappingcapability of the catalyst is reduced.

The term “dissociation” used herein means that the NOx trapped by thecatalyst with adsorption, absorption, occlusion and other reactions isdetached from the catalyst when the NOx is reduced in the stoichiometricor rich state. With regards to a NOx absorbing type catalyst, the word“release” is often used as representing the meaning in contrast with“absorption”, and the “release” is also included in the category of the“dissociation”.

In an exhaust gas purifying apparatus provided with a lean NOx catalysthaving a honeycomb substrate which totally does not cause or hardlycauses the alkali attack phenomenon, it is an object of the presentinvention to overcome the problem of dissociation of NOx occurred whenan operation atmosphere is changed over to reduce trapped NOx.

To achieve the above object, an exhaust gas purifying apparatusaccording to the present invention includes, in an exhaust passage of aninternal combustion engine, a lean NOx catalyst having the function oftrapping NOx in a lean atmosphere under coexistence of oxygen andincluding a catalyst layer that has the function of reducing the trappedNOx by a reductant in a rich or stoichiometric atmosphere and issupported on a honeycomb substrate having a low affinity with componentsof the catalyst layer, and a NOx reducing catalyst disposed downstreamof the lean NOx catalyst and having the function of reducing NOx by areductant in the rich or stoichiometric atmosphere.

In the present invention, the term “catalyst layer” means a state inwhich catalyst active components including a NOx trapping component anda NOx reducing component are supported on a support made of, e.g.,alumina.

The exhaust gas purifying apparatus of the present invention may furthercomprise a pre-catalyst, which is constituted as a three-way catalyst,upstream of the lean NOx catalyst. Alternatively, the exhaust gaspurifying apparatus may further comprise a filter for trapping andremoving particulates upstream or downstream of the lean NOx catalyst.

A exhaust gas purifying method of the present invention comprises thesteps of trapping NOx contained in the exhaust gas in the leanatmosphere and removing the NOx from the exhaust gas by the lean NOxcatalyst having the above-described structure, and then changing over anoperation atmosphere to the rich or stoichiometric atmosphere, therebyreducing NOx dissociated from the lean NOx catalyst with the changeoverof the operation atmosphere into nitrogen (N₂) by the NOx reducingcatalyst.

The present invention is applicable to any kind of lean NOx catalystthat causes a phenomenon similar to the alkali attack phenomenon. Forexample, when a catalyst contains, as the NOx trapping component, analkaline-earth metal element such as strontium (Sr) and barium (Ba), thealkaline-earth metal element causes an attack phenomenon against thehoneycomb substrate in a similar way although the attack phenomenon isnot so strong as that caused by alkali metal. The present invention canbe applied to such a catalyst as well.

According to the present invention, by employing, as the honeycombsubstrate, a material that has a low affinity with the NOx trappingcomponent, when a lean NOx catalyst supporting the NOx trappingcomponent thereon in an increased amount is used, it is possible toovercome the problem that a part of trapped NOx is dissociated when thetrapped NOx is reduced in the rich or stoichiometric atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a change of a NOx purification rate when theoperation mode is changed over from the lean operation to thestoichiometric operation;

FIG. 2 is a schematic view showing a basic construction of an exhaustgas purifying apparatus according to the present invention;

FIG. 3 is a graph showing results of Test Example 1;

FIG. 4 is a graph showing results of Test Example 2;

FIG. 5 is a graph showing results of Test Example 3;

FIG. 6 shows another embodiment of the present invention and is aschematic view of an exhaust gas purifying apparatus including apre-catalyst;

FIG. 7 is a graph showing results of Test Example 4 conducted for theexhaust gas purifying apparatus including the pre-catalyst;

FIG. 8 shows still another embodiment of the present invention and is aschematic view of an exhaust gas purifying apparatus including a DPF(Diesel Particulate Filter);

FIG. 9 is a sectional view of the DPF as viewed in the direction of flowof exhaust gas; and

FIG. 10 is a schematic view of an exhaust gas purifying apparatusaccording to still another embodiment of the present invention, in whichlayout of the DPF is changed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail.

In the following description, an attack phenomenon caused by drift of aNOx trapping component is called an alkali attack for the sake ofconvenience. Also, a step of temporarily changing over the operationmode to the stoichiometric or rich operation during the lean operationis called a rich spike.

FIG. 1 is a graph showing a time-dependent change of a NOx purificationrate, particularly at the time when the operation mode is changed overfrom the lean operation to the stoichiometric operation, for a lean NOxcatalyst in which the occurrence of the alkali attack is suppressed byemploying, as a honeycomb substrate, a material that has a low affinitywith components of a catalyst layer.

The lean NOx catalyst traps NOx contained in exhaust gas during the leanoperation of an internal combustion engine. Because the NOx trappingcapability is finite, the NOx purification rate lowers as the amount ofNOx trapped by the lean NOx catalyst increases.

When the amount of the trapped NOx reaches a certain value, theoperation mode of the internal combustion engine is temporarily changedover to the stoichiometric or rich combustion mode so that the trappedNOx is reduced for purification. In other words, the rich spike isperformed.

The rich spike causes, on the lean NOx catalyst, a reaction between thetrapped NOx and a reducing gas component in the exhaust gas, whereby theNOx is reduced to N₂.

Because the NOx reduction reaction is exothermic, a part of the trappedNOx may dissociate from the lean NOx catalyst while it remains notreduced, and a negative purification rate may occur transiently.

The phenomenon of dissociation of the trapped NOx becomes morenoticeable when the exhaust gas is at a high temperature level at whichthe NOx trapping capability lowers. Also, that phenomenon becomes morenoticeable when the amount of the NOx trapping component in the catalystlayer is increased and the lean operation time is prolonged to increasethe amount of the trapped NOx.

Thus, even in the case using the honeycomb substrate that does not causethe alkali attack, there is naturally a limit in the amount of an alkalimetal, i.e., the NOx trapping component, supported on the catalyst fromthe viewpoint of avoiding such a transient phenomenon, as describedabove.

In the present invention, the alkali attack is suppressed by using thehoneycomb substrate that has a low affinity with the components of thecatalyst layer, and a NOx reducing catalyst is disposed downstream ofthe lean NOx catalyst to reduce the NOx having dissociated from the leanNOx catalyst, thereby eventually increasing the amount of the NOxtrapping component.

FIG. 2 shows a basic construction of an exhaust gas purifying apparatusaccording to the present invention.

A lean NOx catalyst 3 and a NOx reducing catalyst 4 are disposed in anexhaust passage 2 of an engine 1, i.e., an internal combustion engine,in this order from the upstream side.

The lean NOx catalyst 3 is constituted such that a catalyst layercontaining an alkali metal and a noble metal is supported on thehoneycomb substrate made of a ceramic that contains, e.g., alumina andcalcium oxide as components. The catalyst layer can further contain anyof alkaline-earth metals such as magnesium (Mg), calcium (Ca), strontium(Sr) and barium (Ba), and rare-earth metals such as cerium (Ce).

The catalyst layer is supported on the honeycomb substrate by coating orany other suitable method. The catalyst layer can be prepared by causingthe above-mentioned catalyst active components to be supported on asupport, which is made of at least one of metal oxides selected fromamong alumina, titanium oxide and zirconium oxide, by impregnation orany other suitable method.

In addition to the above-mentioned ceramic honeycomb, the honeycombsubstrate can also be constituted as a metal honeycomb that ismanufactured by forming metal foils made of primarily stainless steelinto a desired shape or by joining the metal foils to each other andthen forming them into the desired shape. When the catalyst layercontains the alkali metal as the NOx trapping component, the alkaliattack can be suppressed by using, as the honeycomb substrate, amaterial that contains no Si.

In the case using the honeycomb substrate which totally does not causeor hardly causes the alkali attack, the amount of the NOx trappingcomponent in the lean NOx catalyst can be increased. On the other hand,such a case raises a problem that the NOx trapping component may driftout of the lean NOx catalyst during the time of the rich spike. Thedrifted-out NOx trapping component may adhere to the NOx reducingcatalyst and may deteriorate the NOx purifying performance of the NOxreducing catalyst.

To cope with the problem that the NOx trapping component drifts out ofthe lean NOx catalyst, the present invention proposes it to set thelinear speed of the exhaust gas flowing through the NOx reducingcatalyst comparable to or preferably higher than the linear speed of theexhaust gas flowing through the lean NOx catalyst. By so setting thelinear speed of the exhaust gas flowing through the NOx reducingcatalyst located on the downstream side, the flow speed of the exhaustgas passing through the NOx reducing catalyst is increased, andtherefore the NOx trapping component becomes hard to adhere to the NOxreducing catalyst correspondingly.

The linear speed of the exhaust gas varies with a change in thecross-sectional area of a flow path in the catalyst. For example, thelinear speed of the exhaust gas flowing through the catalyst on thedownstream side can be increased by changing a catalyst outer diametersuch that the cross-sectional area of the catalyst on the downstreamside is smaller than the cross-sectional area of the catalyst on theupstream side.

As an alternative, when the catalyst on the upstream side and thecatalyst on the downstream side have the same outer diameter, the linearspeed of the exhaust gas can be varied by changing a porosity of thehoneycomb. When the number of cells per unit area is the same, theporosity reduces as the thickness of a cell wall is increased. Thesmaller the porosity, the higher is the linear speed of the exhaust gas.

When the amount of the alkali metal or the alkaline-rare metal supportedon the lean NOx catalyst exceeds 20 g per bulk volume of the honeycomb,the trapped NOx cannot be often completely purified because of the largeamount of the trapped NOx, even if the air-fuel ratio during the time ofthe rich spike is set to a lower value to increase the NOx reducingcapability. The not completely purified NOx is more apt to dissociate.The present invention is, however, free from such a problem because theNOx reducing catalyst is disposed downstream of the lean NOx catalystand the dissociated NOx is reduced by the NOx reducing catalyst. Thus,the present invention can be said as providing a method that iseffective for the case of increasing the amount of the alkali metal orthe alkaline-rare metal supported on the lean NOx catalyst over 20 g perbulk volume of the honeycomb, to thereby increase the amount of NOx tobe trapped.

Note that the NOx reducing catalyst 4 is preferably made of a three-waycatalyst, but it may be a lean NOx catalyst instead.

FIG. 6 shows an exhaust gas purifying apparatus according to anotherembodiment of the present invention in which a pre-catalyst 5 made of athree-way catalyst is disposed in the exhaust passage 2 upstream of thelean NOx catalyst 3. The provision of the pre-catalyst 5 increases thecapability of purifying hydrocarbons (HC).

FIGS. 8 to 10 show exhaust gas purifying apparatuses according to otherembodiments of the present invention in which a filter capable oftrapping and removing particulates, i.e., a diesel particulate filter 6(hereinafter referred to as a “DPF”), is additionally disposed. In theexhaust gas purifying apparatus of FIG. 8, the DPF 6 is disposedupstream of the lean NOx catalyst 3. In the exhaust gas purifyingapparatus of FIG. 10, the DPF 6 is disposed between the lean NOxcatalyst 3 and the NOx reducing catalyst 4. FIG. 9 shows a cross-sectionof the DPF 6 in FIG. 8. These embodiments are effective in a dieselengine. Stated another way, since the DPF 6 serves to remove sootcontained in diesel exhaust gas, it is possible to avoid a reduction inthe NOx purifying performance of the catalyst disposed on the downstreamside.

The DPF 6 can be constituted of, e.g., a monolithic honeycomb filterthat is formed by alternately sealing off inlets and outlets of a porouscordierite honeycomb. Another practicable example of the DPF 6 is aceramic fiber laminated filter prepared by wrapping ceramic fibersaround a porous tube. Still other practicable examples of the DPF 6 area filter prepared by forming a metal wire mesh into a hollow cylindricalshape, or a filter prepared by laminating sintered metal sheets oneabove another.

The arrangement of FIG. 8 can provide an additional advantage that sincethe particulates are less accumulated in the lean NOx catalyst and theNOx reducing catalyst, both the lean NOx catalyst and the NOx reducingcatalyst can be prolonged in service life. The arrangement of FIG. 10can provide an additional advantage that since the concentration of theexhaust gas is not averaged by the DPF 6 during the time of the richspike, the rich level is less apt to become shallow and hence theair-fuel ratio is less apt to become high, whereby the trapped NOx canbe more easily purged.

[EXAMPLE]

The present invention will be described in more detail below inconnection with an example. In the following example, the lean NOxcatalyst had a bulk volume of 0.71 L, and the honeycomb had an outerdiameter of 105.7 mmφ and a length of 81.2 mm. Also, the lean NOxcatalyst, the NOx reducing catalyst, and the pre-catalyst were mountedin a test vehicle after being subjected to thermal endurance treatmentat 850° C. for 50 hours in advance.

(Preparation of Lean NOx Catalyst A)

The honeycomb substrate used in the example was a ceramic honeycomb madeof alumina and calcium oxide and having cells in nominal number of 400cells/inch² (about 62 cells/cm²). An alumina slurry made of alumina andnitrate acidic alumina was coated on the ceramic honeycomb such that theamount of alumina was 190 g per bulk volume. Then, an alumina coatedhoneycomb was obtained through steps of drying and firing.

The alumina coated honeycomb was impregnated with a solution of cerium(Ce) nitrate and was subjected to drying at about 100° C. and then tofiring for 1 hour at about 600° C.

Subsequently, the alumina coated honeycomb was impregnated with a mixeddipping liquid, i.e., a mixed solution of sodium (Na) nitrate, magnesium(Mg) nitrate, titania sol, and dinitrodiammine-platinum (Pt) nitrate,followed by drying at about 100° C. and then firing for 1 hour at about600° C.

With the process described above, a lean NOx catalyst A according to theexample of the present invention was obtained in which catalyst activecomponents were supported in amounts of Ce: 27 g, Na: 50 g, Mg: 1.8 g,Ti: 4 g, and Pt: 3 g per bulk volume of the honeycomb.

(Preparation of Lean NOx Catalyst B)

A lean NOx catalyst B according to the example of the present inventionwas prepared exactly in the same manner as the lean NOx catalyst Aexcept for using, as the honeycomb substrate, a metal honeycomb made ofprimarily stainless steel and having cells in nominal number of 400cells/inch² (about 62 cells/cm²). The amounts of the catalyst activecomponents per bulk volume of the honeycomb were the same as those inthe lean NOx catalyst A.

(Preparation of Lean NOx Catalyst C)

A lean NOx catalyst C according to a comparative example was preparedexactly in the same manner as the lean NOx catalyst A except for using,as the honeycomb substrate, a honeycomb made of cordierite and havingcells in nominal number of 400 cells/inch² (about 62 cells/cm²). Theamounts of the catalyst active components per bulk volume of thehoneycomb were the same as those in the lean NOx catalyst A.

(Preparation of NOx Reducing Catalyst X)

An alumina coated honeycomb having an alumina amount of 100 g per bulkvolume of the honeycomb was prepared by coating an alumina slurry on ahoneycomb made of cordierite and having cells in nominal number of 400cells/inch² (about 62 cells/cm²), and by subjecting the honeycomb tosteps of drying and firing. Additionally, the alumina slurry used hereinwas the same as that used in preparing the lean NOx catalyst A.

The alumina coated honeycomb was impregnated with a solution of cerium(Ce) nitrate, and was subjected to drying at about 100° C. and then tofiring for 1 hour at about 600° C. Thereafter, the alumina honeycomb wasimpregnated with a mixed solution of dinitrodiammine-platinum (Pt)nitrate and rhodium (Rh) nitrate, followed by steps of drying andfiring.

In such a way, a NOx reducing catalyst X was obtained in which catalystactive components were supported in amounts of Ce: 27 g, Pt: 2 g, andRh: 0.2 g per bulk volume of the honeycomb.

(Preparation of NOx Reducing Catalyst Y)

A NOx reducing catalyst Y was prepared exactly in the same manner as theNOx reducing catalyst X except for using, as the honeycomb substrate, ahoneycomb made of cordierite and having an outer diameter of 86 mmφ, alength of 122 mm, a bulk volume of 0.71 L, and cells in nominal numberof 400 cells/inch² (about 62 cells/cm²). This NOx reducing catalyst Yhad the same volume as the NOx reducing catalyst X, but the honeycombcross-sectional area of the NOx reducing catalyst Y was 2/3 time that ofthe NOx reducing catalyst X, namely the linear speed of the exhaust gasin the catalyst Y is 1.5 times that in the catalyst X.

(Preparation of NOx Reducing Catalyst Z)

A NOx reducing catalyst Z was prepared exactly in the same manner as theNOx reducing catalyst X except for using, as the honeycomb substrate, ahoneycomb made of cordierite and having an outer diameter of 86 mmφ, alength of 122 mm, a bulk volume of 0.71 L, and cells in nominal numberof 600 cells/inch² (about 93 cells/cm²). This NOx reducing catalyst Zgave the same linear speed of the exhaust gas as that in the NOxreducing catalyst Y, but it had the number of cells 1.5 times that inthe latter.

(Preparation of Pre-Catalyst)

An alumina coated honeycomb having an alumina amount of 100 g per bulkvolume of the honeycomb was prepared by coating the same alumina slurryas that used in preparing the lean NOx catalyst A on a honeycomb made ofcordierite and having a bulk volume of 0.3 L, and by subjecting thehoneycomb to steps of drying and firing.

The alumina coated honeycomb was impregnated with a solution of cerium(Ce) nitrate, and was subjected to drying at about 100° C. and then tofiring at about 600° C. Thereafter, the alumina honeycomb wasimpregnated with a mixed solution of dinitrodiammine-platinum (Pt)nitrate, rhodium (Rh) nitrate, and dinitrodiammine-palladium (Pd)nitrate, followed by drying at about 100° C. and then firing at about600° C.

In such a way, a pre-catalyst (three-way catalyst) was obtained in whichcatalyst active components were supported in amounts of Ce: 27 g, Pt:1.5 g, Rh: 0.15 g, and Pd: 5 g per bulk volume of the honeycomb.

[TEST EXAMPLE 1]

Test Example 1 was conducted to examine respective NOx purifyingperformances of the lean NOx catalysts A, B and C.

First, the lean NOx catalyst A was mounted in an exhaust pipe of a leanburn vehicle (test vehicle) having a displacement of 2.0 L.

Then, the test vehicle was held fixed on a chassis dynamometer and wasoperated to run at a constant speed on condition that the air-fuel (A/F)ratio was set to 20 and the vehicle speed was set to 40 km/h (with thetemperature at the catalyst inlet being about 300° C).

During the run, the operation mode of the test vehicle was changed overfrom the lean burn operation to the rich operation to perform the richspike. After 1 minute from the changeover to the rich operation, the NOxconcentration at an engine outlet (i.e., the NOx concentration at acatalyst inlet) and the NOx concentration at an exhaust pipe outlet(i.e., the NOx concentration at a catalyst outlet) were measured and aNOx purification rate (%) was computed based on the following formula(1): NOx purification rate (%)=(NOx concentration at engine outlet−NOxconcentration at exhaust pipe outlet)+NOx concentration at engineoutlet×100 (1)

A similar test was conducted for the case of running the test vehicle ata constant vehicle speed of 70 km/h (with the temperature at thecatalyst inlet being about 400° C).

Further, those tests were likewise repeated for each of the lean NOxcatalysts B and C in place of the lean NOx catalyst A.

Test results are shown in FIG. 3. When each of the lean NOx catalysts Aand B is solely used, high NOx purifying performance in match with itshigh NOx trapping capability is not obtained. The NOx purifyingperformance of the lean NOx catalyst C employing the cordieritehoneycomb is more inferior to those of the lean NOx catalysts A and B.The reason why the NOx purifying performance of the lean NOx catalyst Cis particularly inferior is presumably in that the amount of the alkalimetal in the catalyst layer is reduced due to the alkali attack. Thehoneycomb strength of the lean NOx catalyst C is also reduced due to thealkali attack at an excessive level.

[TEST EXAMPLE 2]

Tests similar to those in Test Example 1 were conducted by disposing thelean NOx catalyst A in the exhaust pipe of the test vehicle on theupstream side, and disposing the NOx reducing catalyst X therein on thedownstream side.

Further, similar tests were repeated for each of the lean NOx catalystsB and C in place of the lean NOx catalyst A. The tests were alsoconducted for the case in which the lean NOx catalyst B was disposed inthe exhaust pipe on the upstream side, and the lean NOx catalyst C wasdisposed therein on the downstream side.

FIG. 4 shows the NOx purifying rates computed based on the above formula(1). Comparing FIGS. 3 and 4 with each other, the advantage resultingfrom the provision of the NOx reducing catalyst can be understood. Morespecifically, with the arrangement in which the NOx reducing catalyst Xor the lean NOx catalyst C is disposed upstream of the lean NOx catalystA or B, the NOx purifying performance is increased. With the arrangementin which the lean NOx catalyst C is disposed on the upstream side,however, the NOx purifying performance is hardly increased even when thelean NOx catalyst X is disposed on the downstream side. The reason whythe arrangements according to the embodiments of the present inventionexhibit relatively high NOx purifying performance is presumably in thatNOx having dissociated during the time of the rich spike are purified bythe NOx reducing catalyst X or the lean NOx catalyst C having the NOxreducing function.

[TEST EXAMPLE 3]

Tests similar to those in Test Example 2 were conducted by disposing thelean NOx catalyst A in the exhaust pipe of the test vehicle on theupstream side, and disposing the NOx reducing catalyst Y or Z therein onthe downstream side in place of the NOx reducing catalyst X.

Test results are shown in FIG. 5. FIG. 5 also shows the test resultsobtained with the combination of the lean NOx catalyst A and the NOxreducing catalyst X in Test Example 2.

There is no significant difference in the NOx purifying performanceamong the NOx reducing catalysts X, Y and Z. Accordingly, it ispreferable to employ the NOx reducing catalyst Y or Z because the linearspeed of the exhaust gas is increased and the alkali metal havingdrifted out of the lean NOx catalyst is less apt to adhere to the NOxreducing catalyst as compared with the case using the NOx reducingcatalyst X.

[TEST EXAMPLE 4]

Tests similar to those in Test Example 1 were conducted by disposing, inthe exhaust pipe of the test vehicle, the pre-catalyst 5, the lean NOxcatalyst 3 and the NOx reducing catalyst 4 in this order from theupstream side, as shown in FIG. 6.

FIG. 7 shows test results of Test Example 4-1 representing an example ofthe present invention which employs the pre-catalyst, the lean NOxcatalyst A and the NOx reducing catalyst X, and Test Example 4-2representing a comparative example which employs the pre-catalyst, thelean NOx catalyst C and the NOx reducing catalyst X.

The exhaust gas temperature at the inlet of the lean NOx catalyst was350° C. at the vehicle speed of 40 km/h, and was 450° C. at the vehiclespeed of 70 km/h.

As seen from FIG. 7, Test Example 4-1 representing the example of thepresent invention exhibits higher NOx purifying performance than TestExample 4-2 representing the comparative example.

Thus, the present invention is able to overcome the problem that, whenthe amount of the NOx trapping component in the lean NOx catalyst isincreased to enhance the NOx trapping capability, a part of trapped NOxis dissociated during the time of the rich spike and the NOx purifyingperformance is deteriorates. As a result, the lean operation time can beprolonged and the fuel consumption can be improved.

1. An exhaust gas purifying apparatus for an internal combustion engine,comprising: a lean NOx catalyst disposed in an exhaust passage of saidinternal combustion engine, having the function of trapping NOx in alean atmosphere under coexistence of oxygen, and including a catalystlayer that has the function of reducing the trapped NOx by a reductantin a rich or stoichiometric atmosphere and is supported on a honeycombsubstrate having a low affinity with components of said catalyst layer;and a NOx reducing catalyst disposed downstream of said lean NOxcatalyst and having the function of reducing NOx by a reductant in therich or stoichiometric atmosphere, wherein said NOx reducing catalystreduces NOx that is dissociated from said lean NOx catalyst when acondition of exhaust gas in said exhaust passage is changed over from alean condition to a rich or stoichiometric condition.
 2. An exhaust gaspurifying apparatus for an internal combustion engine according to claim1, wherein said lean NOx catalyst contains a NOx trapping component anda NOx reducing component in said catalyst layer.
 3. An exhaust gaspurifying apparatus for an internal combustion engine according to claim1, wherein said lean NOx catalyst is a three-way catalyst.
 4. An exhaustgas purifying apparatus for an internal combustion engine according toclaim 1, wherein said lean NOx catalyst contains, as said NOx trappingcomponent, at least one element selected from among alkali metals andalkaline-earth metals, and said honeycomb substrate contains no Si as acomponent thereof.
 5. An exhaust gas purifying apparatus for an internalcombustion engine according to claim 4, wherein the NOx trappingcomponent in said lean NOx catalyst contains at least one elementselected from among Li, Na, K, Cs, Sr and Ba, and an amount of the NOxtrapping component supported on said honeycomb substrate is not smallerthan 20 g per bull volume of said honeycomb substrate.
 6. An exhaust gaspurifying apparatus for an internal combustion engine according to claim4, wherein said lean NOx catalyst contains a noble metal as the NOxreducing component.
 7. An exhaust gas purifying apparatus for aninternal combustion engine according to claim 1, wherein said lean NOxcatalyst and said NOx reducing catalyst are set to have a relativerelationship therebetween such that a linear speed of the exhaust gasflowing through said NOx reducing catalyst is comparable to or higherthan a linear speed of the exhaust gas flowing through said lean NOxcatalyst.
 8. An exhaust gas purifying apparatus for an internalcombustion engine according to claim 7, wherein said NOx reducingcatalyst is constituted by a honeycomb substrate having a catalyst layersupported thereon, and said honeycomb substrate of said NOx reducingcatalyst has a smaller porosity than said honeycomb substrate of saidlean NOx catalyst.
 9. An exhaust gas purifying apparatus for an internalcombustion engine according to claim 1, further comprising a three-waycatalyst disposed in said exhaust passage upstream of said lean NOxcatalyst.
 10. An exhaust gas purifying apparatus for an internalcombustion engine, comprising an exhaust gas purifying catalyst and afilter trapping and removing particulates which are disposed in anexhaust passage of said internal combustion engine, wherein said exhaustgas purifying catalyst comprises a lean NOx catalyst having the functionof trapping NOx in a lean atmosphere under coexistence of oxygen andincluding a catalyst layer that has the function of reducing the trappedNOx by a reductant in a rich or stoichiometric atmosphere and issupported on a honeycomb substrate having a low affinity with componentsof said catalyst layer, and a NOx reducing catalyst having the functionof reducing NOx by a reductant in the rich or stoichiometric atmosphere;said lean NOx catalyst is disposed upstream or downstream of saidfilter, and said NOx reducing catalyst is disposed downstream of saidlean NOx catalyst and said filter; and said NOx reducing catalystreduces NOx that is dissociated from said lean NOx catalyst when acondition of exhaust gas in said exhaust passage is changed over fromthe lean atmosphere to the rich or stoichiometric atmosphere.
 11. Anexhaust gas purifying apparatus for an internal combustion engineaccording to claim 10, wherein said lean NOx catalyst is disposeddownstream of said filter, and said NOx reducing catalyst is disposeddownstream of said lean NOx catalyst.
 12. An exhaust gas purifyingapparatus for an internal combustion engine according to claim 10,wherein said lean NOx catalyst is disposed upstream of said filter, andsaid NOx reducing catalyst is disposed downstream of said filter.
 13. Anexhaust gas purifying apparatus for an internal combustion engineaccording to claim 10, wherein said lean NOx catalyst contains, as saidNOx trapping component, at least one element selected from among alkalimetals and alkaline-earth metals, and said honeycomb substrate containsno Si as a component thereof.
 14. An exhaust gas purifying apparatus foran internal combustion engine according to claim 13, wherein said leanNOx catalyst contains a noble metal as the NOx reducing component. 15.An exhaust gas purifying apparatus for an internal combustion engineaccording to claim 10, wherein said NOx reducing catalyst is athree-way.
 16. An exhaust gas purifying apparatus for an internalcombustion engine according to claim 10, wherein said lean NOx catalystand said NOx reducing catalyst are set to have a relative relationshiptherebetween such that a linear speed of the exhaust gas flowing throughsaid NOx reducing catalyst is comparable to or higher than a linearspeed of the exhaust gas flowing through said lean NOx catalyst.
 17. Anexhaust gas purifying apparatus for an internal combustion engineaccording to claim 16, wherein said NOx reducing catalyst is constitutedby a honeycomb substrate having a catalyst layer supported thereon, andsaid honeycomb substrate of said NOx reducing catalyst has a smallerporosity than said honeycomb substrate of said lean NOx catalyst.
 18. Anexhaust gas purifying method for use in an internal combustion engine topurify exhaust gas exhausted from said internal combustion engine by anexhaust gas purifying catalyst disposed in an exhaust passage, themethod comprising the steps of: trapping NOx contained in the exhaustgas from said internal combustion engine, which is operated in a leanatmosphere under coexistence of oxygen, and removing the NOx from theexhaust gas by a lean NOx catalyst having the function of trapping theNOx in a lean atmosphere and including a catalyst layer that has thefunction of reducing the trapped NOx by a reductant in a rich orstoichiometric atmosphere and is supported on a honeycomb substratehaving a low affinity with components of said catalyst layer; andtemporarily changing over an operation mode of said internal combustionengine to operation in the rich or stoichiometric atmosphere, andreducing NOx dissociated from said lean NOx catalyst with the changeoverof the operation mode by a NOx reducing catalyst having the function ofreducing NOx by a reductant in the rich or stoichiometric atmosphere.19. An exhaust gas purifying method for an internal combustion engineaccording to claim 18, wherein a linear speed of the exhaust gas flowingthrough said NOx reducing catalyst is comparable to or higher than alinear speed of the exhaust gas flowing through said lean NOx catalyst.